Electrophysiological diagnosis of cervical OPLL myelopathy

Abstract

Introduction It is extremely diffi cult to make a correct diagnosis of the responsible lesion in multilevel continuous- and mixed-type ossification of the posterior longitudinal ligament (OPLL) even after magnetic resonance imaging (MRI). Understanding the function of the preoperative spinal cord is crucial for surgical planning and predicting postoperative recovery. Also, intraoperative spinal cord monitoring is required for cervical OPLL surgery because OPLL patients show postoperative neurological deterioration more frequently than any other pathogenesis, such as cervical spondylotic myelopathy. Method Six spinal cord monitoring systems may be used (Fig. 1a,b). 1. Sp-SCEPs (spinal cord evoked potentials elicited by spinal cord stimulation) 2. Pn-SCEPs (spinal cord evoked potentials elicited by peripheral nerve stimulation) 3. Br(M)-SCEPs (spinal cord evoked potentials elicited by transcranial magnetic stimulation) 4. Br(M)-MsEPs (muscle evoked potentials elicited by transcranial magnetic stimulation) 5. Br(E)-SCEPs (spinal cord evoked potentials elicited by transcranial electrical stimulation) 6. Br(E)-MsEPs (muscle evoked potentials elicited by transcranial electrical stimulation) The recording electrode for SCEPs consists of a polyethylene tube with fi ve platinum bands with diameters of 0.75 mm spaced 15 mm apart, which corresponds with the height of the vertebral body. Five waves can be recorded simultaneously using this fi ve-pole electrode, which simplifi es calculation of the conduction velocity (Fig. 1c). The recording electrode is pushed upward until the fi ve poles span the OPLL and until at least the top pole is located above the OPLL. The recording electrode should be placed at the center of the posterior spinal canal to record the Pn-SCEPs equally from the bilateral upper extremities. A critical safety factor during the insertion is to do it with the patient fully conscious, allowing termination of the procedure if the subject complains of severe or radiating pain and thereby preventing neural injury. In cases in which it is diffi cult to pass the electrode, another electrode is inserted above the narrowed OPLL area. A stimulating electrode for Sp-SCEPs is placed at the lower thoracic epidural space, and a reference electrode is placed at the subcutaneous tissue in the neck. Sp-SCEPs are recorded from the cervical epidural space after electrical stimulation (0.3 ms, 15 mA) of the lower thoracic spinal cord by epidural electrodes (0.75 mm). Pn-SCEPs are obtained after stimulation of the peripheral nerves in the upper extremity (0.2 ms, twice the threshold current) (Nihon-Kohden, Tokyo, Japan). In addition, to diagnose the upper border of the spinal disorders, Br(M)-SCEPs [1] are recorded after transcranial magnetic stimulation of the brain because ascending Sp-SCEPs in the presence of severe myelopathy disappear in the OPLL area. Electrical stimulation cannot be applied to the skull surface in an awake condition because of severe pain [2]. In an awake condition, the motor area is stimulated by magnetic stimulation using a double-cone coil for the lower extremity and an eight-shaped coil for the upper extremity (MagStim, London, UK) instead of electrical stimulation [1,3]. Br(E)-MsEPs, Br(E)-SCEPs, and Sp-SCEPs are used for intraoperative spinal cord monitoring. Total intravenous anesthesia (TIVA: propofol, fentanyl) is used for intraoperative monitoring because inhalation anesthesia makes it diffi cult to record evoked muscle action potentials. TIVA is used so synaptic activities at the anterior horn of the spinal cord are not inhibited. Muscle relaxant (vecuronium bromide 0.02-0.04 mg/ kg/h) is also administered to regulate muscle contraction, making operative procedures possible. Electrical stimulation (200 mA, 0.5-ms pulse) is applied to the skull surface with two needle electrodes installed 5 cm lateral and 2 cm anterior to the Cz for Br(E)-MsEP and Br(E)-SCEP recording. The brain at an anode site is relatively stimulated; therefore, if right anode stimulation is applied, spinal cord evoked poten tials and muscle evoked potentials of the left side can be easily recorded. A single stimulation of 3 Hz is applied for Br(E)-SCEPs, and fi ve train stimulations of 1 Hz are applied for Br(E)-MsEPs [4,5]. Spinal cord evoked potentials elicited by transcranial stimulation consist of direct (D) waves and indirect (I) waves depending on whether the motor area is directly or indirectly stimulated. The D wave is elicited by electrical stimulation and is not affected even under general anesthesia. The signifi cance of this method is apparent during intraoperative monitoring for the diagnosis of descending tracts. In severe myelopathy cases, Br(E)-MsEPs cannot be recorded (3.0%), although spinal cord stimulation by a stimulating electrode above the responsible lesion commonly facilitates recordings of evoked muscle potentials [6]. Sp-SCEPs were found to originate from the dorsal and posterolateral funiculus. Pn-SCEPs have also been reported to originate from the dorsal nerve root potentials and postsynaptic action potentials in the posterior horns. © Springer 2006.